Power conversion devices that provide power from a storage battery to an AC load are often used in backup power supply devices such as UPS (Uninterruptible Power Supply) (for example, refer to Patent Literature 1 (FIG. 1)). Such a power conversion device includes a DC/DC converter for stepping up a voltage of the storage battery, and an inverter for performing conversion from DC to AC.
In this case, the DC/DC converter steps up the voltage of the storage battery to a constant level equal to or higher than a required AC peak value through high-frequency switching, and outputs the stepped-up voltage to a DC bus. The inverter modulates the constant voltage so as to have an AC waveform through high-frequency switching. Therefore, the DC/DC converter and the inverter each always perform high-frequency switching.
Normally, the power conversion device has a bidirectional property, and is capable of converting an AC voltage outputted from an AC power supply such as a commercial power supply to a DC voltage suitable for charging, and charging the storage battery. In this case, the inverter operates as an AC/DC converter, and full-wave rectifies the AC waveform. The full-wave-rectified waveform is smoothed by a capacitor. Meanwhile, the DC/DC converter exerts a step-down function. The AC/DC converter can perform the full-wave rectification by using diodes connected in reverse-parallel to switching elements. In this case, four switching elements forming the AC/DC converter are kept OFF.
In the power conversion device, when discharging the storage battery, the DC/DC converter and the inverter each always perform high-frequency switching. Due to the high-frequency switching, the corresponding switching loss occurs, and the switching loss causes deterioration in conversion efficiency. In order to reduce such switching loss to enhance the conversion efficiency, for example, a control method can be adopted in which the DC/DC converter and the inverter are caused to alternately perform switching operations within one cycle of AC (for example, refer to Patent Literature 2). In this case, the number of times of switching in the power conversion device as a whole can be reduced. The control method for causing the DC/DC converter and the inverter to alternately perform switching operations within one cycle of AC as described above can be similarly executed in a reverse direction.